1. Field of the Invention
The United States produces more than 5 billion lbs. of plastic film each year, virtually all of which is made from petroleum-based raw materials. These films are most economically made by an economical extrusion blowing process in which a tubular extruded bubble is expanded and shaped by air streams at the die exit. In the field of agriculture, approximately 130 million lbs. of polyethylene (PE) film is used annually as mulch to improve crop yields by controlling weeds, retaining soil moisture, and reducing nutrient leaching. Since PE mulch cannot be reused and does not degrade between growing seasons, it must be removed from the field and disposed at a current estimated cost of $100 per acre. Other agricultural uses for plastic film include seedling containers and the protection of roots during transplanting. These films have also become an important factor in the packaging of consumer products and as containers for the disposal of waste.
Rapidly increasing prices, dwindling supplies of petroleum, and the need for economically feasible biodegradable films that do not adversely affect the environment upon disposal have intensified the need for alternate sources of raw materials for making plastics. This invention relates to the preparation of such films by the blowing of formulations based upon a renewable resource.
2. Description of the Prior Art
Numerous attempts have been made to produce degradable films from petroleum and cellulose-derived materials [Chemical Week 109: 45-46 (1971)] including PE-coated paper ]Chemical Week 110: 44 (1972)] and polybutene-1 films (U.S. Pat. No. 3,590,528). None has been completely successful, apparently because they were too costly, or they decompose too slowly for many applications. Starch is probably the most abundant, low-cost, biodegradable polymer available and its use in plastic film production would greatly reduce the demand for petrochemicals and the negative impact on the environment now caused by discarding nonbiodegradable films. Since starch alone forms a brittle film that is sensitive to water, it is generally understood that starch must be combined with other materials in order to produce a satisfactory product. PE is the most widely used material for producing films that have desirable physical properties for packaging and mulch applications, and it is available at a relatively low cost. It is therefore a particularly desirable material to combine with starch to achieve the desired flexibility, water resistance, and strength. However, previous attempts to produce blown films from compositions containing high levels of starch combined with PE have been unsuccessful.
Griffin (U.S. Pat. No. 4,016,117) teaches that about 8% predried starch (0.5% moisture), 90% PE, 1.6% ethyl oleate, and less than 1% oleic acid compositions can be converted to blown films (Example I). However, essentially the same composition could not be blown into a satisfactory film if the starch contained as much as 2% moisture (paragraph bridging columns 3 and 4; Example VII). The product became disfigured and weakened by the presence of numerous small bubbles created by the conversion of the free moisture to steam. This limitation on the moisture content requires special drying, handling, and storage techniques preparatory to film formation. Griffin further observed that both gelatinizing the starch (column 3, lines 36-39) and increasing the starch content of film formulations from 5 to 15% (Example XI) resulted in feel and crease retention properties much more paperlike than unmodified PE film.
The discovery by Otey et al. (U.S. Pat. No. 4,133,784) that compositions of ethylene arylic acid copolymer (EAA) and a starchy material can be formed into films that are flexible, water resistant, heat sealable, and biodegradable has intensified interest in the possibility of making starch-based films. These films were formed by either casting, simple extruding, or milling the starch-EAA composition. All are relatively slow processes that are considerably more expensive than the more conventional extrusion blowing technique. The relatively high processing cost coupled with the high price of EAA compared to PE tend to diminish this composition's potential for achieving large-scale commercial success. Also, at certain starch levels needed for achieving desired mechanical properties, the optimum degrees of biodegradability and UV stability are compromised.
Our attempts to incorporate pelletized PE into the pelletized EAA and starch composition described by Otey et al. (U.S. Pat. No. 4,133,734) and to convert the composite into blown films were not successful. Continuous blowing was difficult because the films ruptured. Visible striations and other evidence of poor compatibility between the starch and resin components were also indicative of an inferior product.